You need propellants to accelerate toward Mars, then to decelerate at Mars, again to re-accelerate from Mars to Earth, and finally to decelerate back at Earth. Accordingly, the mass of these required propellants, in short, drives our need for innovative launch vehicles.
Buzz AldrinNobody ever asks who was the seventh person on the Moon. The only thing they know is who's number one and who's number two. Does anybody know who the last man was?
Buzz Aldrin